The present invention relates to horizontal boring machines for boring cylindrical surfaces having horizontal axes and axially separated from each other, such as the seats of an engine crankshaft in the crankcase of an internal combustion engine.
In particular, the invention refers to horizontal boring machines of the type including a boring bar driven in rotation by a chuck and carrying at least one cutting bit, during means for driving rotation of said chuck, means for axially moving the group composed of the chuck and the associated driving means, a counter-bar coupled in rotation head-to-head with said boring bar and driven in rotation by a respective auxiliary chuck, driving means for driving rotation of the auxiliary chuck in synchronism with the rotation of the boring bar, and means for axially moving the group composed of said counter-bar and said driving means for driving rotation of the counter-bar in synchronism with the axial movement of the boring bar, said boring bar being provided with a device for adjusting the radial position of said at least one cutting bit that is associated therewith.
A boring machine of the above-specified type is described and illustrated in U.S. Pat. No. 6,343,899 B1 and in the corresponding EP 0 968 069 B1.
FIG. 1 of the enclosed drawings is a reproduction of FIG. 2 of the above-identified U.S. patent. In this figure, reference number 2 indicates the crankcase of an internal combustion engine carrying five cylindrical seats, axially spaced apart from each other and which are to define the seats for the engine crankshaft. The mechanical machining of these seats is carried out using the machine tool, generally indicated by reference number 1 in FIG. 1, which includes a bed 6 with a worktable 8 on which the crankcase 2 is fixed. The boring work on these seats is performed using a boring bar 20, carrying at least one radially adjustable cutting bit (not visible in FIG. 1), which is driven in rotation by a chuck 24, in turn driven by a motor 32 via a transmission 34. The entire group composed of the boring bar 20, the associated chuck 24 and the associated motor 32 is carried on a slide 18 that can move axially on guides 17 using a screw system controlled by a motor 22. On the group carried by the slide 18, a motor 38 is also mounted for driving, via a transmission 40, the axial movement of a shaft 36 inside the boring bar 20, for the purpose of providing the radial adjustment of the cutting bit carried by the bar 20. The motor 32 is typically an electric motor.
The end of the boring bar 20 is coupled in rotation, head to head, with the end of a counter-bar 46 that is driven in rotation by a chuck 42, in turn driven by an electric motor 50, which is slave to the motor 32 to ensure the control of synchronous rotation for the boring bar 20 and the counter-bar 46. A transmission 52 connects the motor 50 to the chuck 42. The entire group composed of the counter-bar 46, the chuck 42 and the motor 50 is carried on a slide 20a that can move axially on guides 16 via a screw system controlled by a motor 22.
At the start of the machining operation, the boring bar 20 and the counter-bar 46 are made to penetrate inside the crankcase 2 from opposite sides until their ends reciprocally engage. When this operation is complete, the group composed of the boring bar 20 and the counter-bar 46 is moved at intervals in the axial direction via the synchronous control of the two slides 18 and 20a, so as to bring, in successive phases, the cutting bit of the boring bar 20 into correspondence with each of the crankshaft seats of the crankcase 2. Each time that the cutting bit 50 is positioned in correspondence with a seat, the synchronous rotation of the boring bar 20 and the counter-bar 46 is activated, together with a progressive axial feed movement to perform the boring of that specific seat. Before and after the machining of each seat, the cutting bit carried by the boring bar 20 is radially withdrawn, via an adjustment system controlled by the motor 38, to allow the new axial positioning of the group without creating interference between the cutting bit and the machined surface or the new surface to be machined. Once the group has been positioned to set up the cutting bit in correspondence with a new seat to be machined, the bit is again radially moved to its operational position, to permit machining. The radial adjustment of the bit is also used for compensating for wear on the bit.
The main drawback of the above-described machine is that it necessitates machining the seats one at a time, which implies a relatively high cycle time. On the other hand, an arrangement with multiple cutting bits axially spaced apart from each other on the boring bar is unthinkable, as in this case it would be impossible in practice to make an automatic radial adjustment system for all of the cutting bits carried by the boring bar.